Vehicles may be fitted with evaporative emission control systems to reduce the release of fuel vapors to the atmosphere. For example, vaporized hydrocarbons (HCs) from a fuel tank may be stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel.
Since leaks in the emissions control system can inadvertently allow fuel vapors to escape to the atmosphere, leak detection routines may be intermittently performed when the engine is not running. Therein, following application of a negative pressure on the fuel system, the system is sealed and a rate of pressure decay is monitored. By comparing the actual pressure decay to a reference value (as determined through a reference orifice), leaks may be identified. In addition, to avoid false positive leak determination, vehicle control systems may abort or delay leak tests if selected conditions are met.
One example approach for reducing false positive leak determination is shown by Suzuki in U.S. Pat. No. 6,973,924. Therein, if refueling of a fuel tank is determined, a leak check routine is delayed until a threshold amount of canister purging has occurred. Specifically, a leak check is not carried out during conditions where a large amount of evaporative fuel is generated due to refueling since the refueling vapors can increase the possibility of a false positive leak determination.
However, the inventors herein have identified potential issues with such an approach. As one example, the approach of Suzuki may not sufficiently address false leak detections occurring due to unintended temporary closing (also referred to as corking) of mechanical fuel tank vent valve(s). In particular, engine-on leak diagnostics may be performed while a vehicle is moving. Therein, the leak diagnostics may be affected by vehicle dynamic maneuvers, such as sweeping turns, climbing of an elevation, or travel along a bumpy road, wherein fuel may slosh and momentarily cork one or more passive tank vent valves (which are otherwise expected to be open during leak diagnostics). When this occurs, the fuel tank may become isolated and the volume of the evaporative system is dramatically reduced. If a leak test is running when the unintended valve closing occurs, false leak detection may occur because leak detection reference pressure values are based on a fuel tank fill volume. As a result, if a fuel tank becomes isolated due to unintended temporary closing of a fuel tank vent valve, the likelihood of false leak detection increases. This reduces the reliability of the leak test while increasing an MIL warranty.
In one example, some of the above issues may be addressed by a method for a vehicle fuel system, comprising: during a fuel system leak test, and in response to unintended temporary closing of a mechanical valve coupled to a fuel tank, not completing the fuel system leak test. Rather, a leak test may be reiterated so that false leak detections are reduced.
As an example, an engine fuel system leak test may be initiated by opening a purge valve. As such, during the leak test, one or more passive, mechanical vent valves coupled to the fuel tank are expected to be open. An engine intake vacuum may then be applied on the fuel system. As vacuum is being pulled down in the fuel tank, the fuel tank pressure may be monitored. A sudden inflection in fuel tank pressure experienced during the (first, or initial) vacuum pull-down may indicate an unintended temporary closing (herein also referred to as corking) and subsequent opening (herein also referred to as uncorking) of a fuel tank vent valve. For example, vacuum may suddenly be pulled down faster than expected, suggested unintended closing of a vent valve, followed by a sudden decrease back to the expected profile, suggested reopening of the vent valve. In one example, the leak test may be performed while a vehicle is moving, and the momentary closing of the vent valve may be induced by certain vehicle maneuvers (e.g., sweeping turns).
In response to the indication of unintended temporary vent valve closing, the fuel system leak test may be discontinued and not completed. Instead, the fuel tank vacuum may be released, the purge valve may be closed and fuel tank settings from prior to the leak test may be resumed. Then, once the fuel tank pressure has stabilized, the fuel system leak test may be re-initiated. Specifically, the purge valve may be re-opened and vacuum may be pulled down again in the fuel tank. If there is no pressure inflection during the (second or subsequent) vacuum pull-down, it may be determined that valve corking did not occur this time around. Accordingly, following the most recent application of vacuum, the fuel tank may be isolated (by closing the purge valve) and vacuum bleed-up to atmospheric pressure may be monitored. A fuel system leak may then be identified based on the rate of vacuum bleed-up. For example, if vacuum bleed-up is faster than a threshold rate, a fuel system leak is confirmed.
In other embodiments, unintended temporary closing of the fuel tank vent valve may be determined due to a pressure inflection experienced during the vacuum bleed-up. For example, vacuum may be bled-up faster than expected, suggested unintended closing of the vent valve, followed by a sudden decrease back to the expected profile, suggested reopening of the vent valve. If the indication is received during the vacuum bleed-up, the fuel system leak test may be discontinued and not completed. That is, the vacuum bleed-up data may be disregarded while initial fuel system settings (those priori to initiating the leak test) are resumed. Then, once the fuel tank pressure stabilizes, the fuel system leak test may be re-initiated. Specifically, vacuum may be pulled down again in the fuel tank. If there is no pressure inflection during the vacuum pull-down and the subsequent vacuum bleed-up, it may be determined that valve corking did not occur this time around. Accordingly, the most recent vacuum bleed-up data may be used to identify a fuel system leak.
In this way, by aborting a fuel system leak test if an unintended momentary closing of a fuel tank vent valve is detected, false leak detections may be reduced. By resuming initial pre-test fuel system settings, and retrying the leak test once fuel tank pressures have stabilized following the aborted leak test, leak tests may be completed with more reliable results. By relying only on vacuum bleed-up data from a leak test when valve corking was not determined, fuel system leaks may be accurately and reliably identified.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.